A new polymer-supported Evans-type chiral auxiliary derived from α-hydroxy-β-amino acid, phenylnorstatine: synthesis and application in solid-phase asymmetric alkylation reactions

Based on a new anchoring strategy, a polymer-supported chiral oxazolidinone was prepared starting from (2R,3S)-3-amino-2-hydroxy-4-phenylbutanoic acid (phenylnorstatine, Pns) and Wang resin. Solid-phase asymmetric alkylation on this resin proceeded in high diastereoselectivity comparable to that of conventional solution-phase model experiments. This study suggests that anchoring through the 5-position of oxazolidinone is highly suited to achieving diastereoselective alkylation reactions on solid-support.     

Practical asymmetric synthesis of a novel γ-secretase inhibitor

An efficient route to γ-secretase inhibitor hydroxyl thiophene sulfonamide 1 is described. The approach contains nine steps with an overall yield of 8%. The synthesis highlights a diastereoselective methylation using Evans' oxazolidinone method and a chiral Strecker reaction via Davis' sulfonimine protocol.     

Chiral oxazolidinones as electrophiles: Intramolecular cyclization reactions with carbanions and preparation of functionalized lactams

The intramolecular cyclizations of oxazolidinones with carbanions adjacent to sulfones, sulfoxides, and phosphonates proceed in high yields to obtain functionalized γ and δ lactams. The chiral oxazolidinone precursors can be readily synthesized from commercial amino acids. The lactams from this study are useful synthetic intermediates, as demonstrated by the synthesis of a precursor for levetiracetam, an antiepileptic drug.     

Chiral Auxiliary Approach for Gold(I)-Catalyzed Cyclizations

Two different classes of stereoselective cyclizations have been developed using a chiral auxiliary approach with commercially available [JohnPhosAu(MeCN)SbF₆] as catalyst. First, a stereoselective cascade cyclization of 1,5-enynes was achieved using the Oppolzer camphorsultam as chiral auxiliary. In this case, a one-pot cyclization-hydrolysis sequence was developed to directly afford enantioenriched spirocyclic ketones. Then, the stereoselective alkoxycyclization of 1,6-enynes was mediated by an Evans-type oxazolidinone. A reduction-hydrolysis sequence was selected to remove the auxiliary to give enantioenriched β-tetralones. DFT studies confirmed that the steric clash between the chiral auxiliary and alkene accounts for the experimentally observed diastereoselective cyclization through the Si face.     

Stereoselective synthesis of orthogonally protected 2,3-disubstituted morpholines using a base-catalysed cascade reaction

The stereoselective synthesis of differentially protected [3-(hydroxymethyl)morpholin-2-yl]methanols is described, starting from chiral epoxides. The key step involves a one-pot oxazolidinone formation via intramolecular epoxide opening and concomitant cyclisation to form the morpholine ring. Selective deprotection reveals the free hydroxymethyl group at either the 2- or 3-position of the morpholine.     

Exploring structural effects of ketone catalysts on asymmetric epoxidation of olefins

Several ketone catalysts containing spiro ethers and lactones have been investigated for the asymmetric epoxidation of olefins. The results showed that substituents on the spiro ring of the ketone catalysts have profound effects on enantioselectivity. Results also suggested that the high enantioselectivities previously observed for conjugated cis-olefins with oxazolidinone containing ketones could be partially due to attractive interactions between the R_π group of the olefin and the carbonyl group of the oxazolidinone. In addition, nonbonding interactions such as van der Waals forces and/or hydrophobic interactions between the olefin substituents and the nitrogen substituents of the oxazolidinone may also be involved in stereodifferentiation. The information gained provides additional understanding of factors important for ketone catalyzed epoxidations.     

Molecular-level chiral discrimination and induction

The review describes the mechanism of chiral discrimination of racemic amines upon crystallization and the induction of chirality in organic reactions by using them as chiral auxiliaries. In order to form conglomerates, which can be resolved into the two enantiomers upon alternative seeding, both formation and packing of 2₁-columns are essentially very important. On the other hand, in order to achieve high efficiency in resolution through diastereomeric salt formation, which is the most practical method, one of a pair of diastereomeric salts derived from a racemic amine and an enantiomerically pure resolving agent should at least have two 2₁-columns and planar boundary surfaces in its crystal structure. On the basis of this knowledge, we developed several artificial chiral auxiliaries such as erythro-2-amino-1,2-diphenylethanol,cis-2-amino-1-acenaphthenol, andcis-2-amino-3,3-dimethyl-1-indanol. These were found to be very efficient chiral auxiliaries in asymmetric inductions: alkylation of chiral imines, catalytic borane-reduction, and alkylation of chiral N-acylated oxazolidinone.     

Synthesis of α-chiral-β,γ-unsaturated carboxylic acid derivatives using chiral auxiliaries

We report an efficient and reliable method for the synthesis of α-chiral-β,γ-unsaturated carboxylic acid derivatives using chiral auxiliaries and vinylacetic acid. Two well-established chiral auxiliaries ((S,S)-pseudoephedrine and (R)-benzyl-oxazolidinone) were chosen to test the merits of this method. Six different electrophiles were examined in the diastereoselective alkylation with both auxiliaries. The pseudoephedrine auxiliary provided isolated yields between 61 and 85% and diastereomeric ratios all greater than 96:4. Employing the same reactions as with the pseudoephedrine derivative, the corresponding oxazolidinone auxiliary provided isolated yields between 0 and 80% with diastereomeric ratios from 80:20 to 93:7.     

Diastereoselective conjugate addition of organocuprates to N-[4-(Dibenzylaminobutenoyl)]oxazolidinone. Synthesis of chiral β-substituted γ-aminoacids

The first conjugate addition reaction of organocuprates to N-enoyl oxazolidinone where a γ-nitrogen atom is present into α,β-unsaturated system is described. The reaction gave syn-products in moderate yields and high diastereomeric ratios. The removal of chiral oxazolidinone moiety and N-deprotection of amino group furnished chiral β-substituted γ -amino acids.     

Asymmetric approach towards the total synthesis of (+)-actinopyllic acid

This paper describes our efforts towards the asymmetric total synthesis of (+)-actinophyllic acid. Starting from the chiral oxazolidinone 9, an azocino [4,3-b]indolyl intermediate (5) possessing the A/B/C ring system and the C16 quaternary stereogenic center of actinophyllic acid has been synthesized. Key steps include a LHMDS-promoted condensation to establish the critical C2–C16 bond and a successive four-step transformation to assemble the eight-membered C-ring of the target molecule.     

Stereoselective conjugate radical additions: application of a fluorous oxazolidinone chiral auxiliary for efficient tin removal

[reaction: see text] A series of asymmetric free-radical-mediated intermolecular conjugate additions using a fluorous oxazolidinone chiral auxiliary has been completed. The fluorous auxiliary facilitated product isolation using fluorous solid phase extractions (FSPE), effectively removing excess organic and organometallic reagents. Parallel reactions carried out with a similar but nonfluorous norephedrine-derived oxazolidinone demonstrated the superior stereoselectivity and purification obtainable with the fluorous chiral auxiliary.     

Diastereoselective alkylations of oxazolidinone vinylogous glycolates

A highly Z-selective isomerization (double bond migration) was observed when oxazolidinone vinylogous glycolate was exposed to a strong base to give N-acyl oxazolidinone, bearing an electron rich olefin. The corresponding enolate was exposed to alkyl halides to provide alkylated compounds on the γ-position with respect to OBn group, with high regioselectivity and moderate diastereoselectivity. However, the nature of the chiral oxazolidinone leads to a significant increase in the reaction diastereoselectivity. A stereospecific formation of cis-olefin was also observed in these alkylated compounds.     

Past,present, and future developments in enantioselective analysis using capillary electromigration techniques

Enantioseparation of chiral products has become increasingly important in a large diversity of academic and industrial applications. The separation of chiral compounds is inherently challenging and thus requires a suitable analytical technique that can achieve high resolution and sensitivity. In this context, CE has shown remarkable results so far. Chiral CE offers an orthogonal enantioselectivity and is typically considered less costly than chromatographic techniques, since only minute amounts of chiral selectors are needed. Several CE approaches have been developed for chiral analysis, including chiral EKC and chiral CEC. Enantioseparations by EKC benefit from the wide variety of possible pseudostationary phases that can be employed. Chiral CEC, on the other hand, combines chromatographic separation principles with the bulk fluid movement of CE, benefitting from reduced band broadening as compared to pressure-driven systems. Although UV detection is conventionally used for these approaches, MS can also be considered. CE-MS represents a promising alternative due to the increased sensitivity and selectivity, enabling the chiral analysis of complex samples. The potential contamination of the MS ion source in EKC-MS can be overcome using partial-filling and counter-migration techniques. However, chiral analysis using monolithic and open-tubular CEC-MS awaits additional method validation and a dedicated commercial interface. Further efforts in chiral CE are expected toward the improvement of existing techniques, the development of novel pseudostationary phases, and establishing the use of chiral ionic liquids, molecular imprinted polymers, and metal-organic frameworks. These developments will certainly foster the adoption of CE(-MS) as a well-established technique in routine chiral analysis.     

Green aspects during synthesis,application and chromatographic analysis of chiral pesticides

The agro-industry uses large quantities of chiral pesticides to mitigate the detrimental effects of pests on crops. Pesticides play a very important role of insuring food security in the world but this benefit may be eroded if principles of green chemistry are not embraced during their synthesis, application and analysis. Commercial chiral pesticide formulations are, usually, synthesised and sold as racemates. The enantiomers of the chiral pesticides in these racemic mixtures usually have enantioselective bioactivities on target organisms. One enantiomer, usually, will be active on the target organism while others are inactive and are discharged into the environment, posing serious pollution problems. This is a serious environmental problem and can be rectified through embracing general principles of green chemistry. This paper reviews the aspects that can enhance greenness during synthesis and the subsequent application of chiral pesticides during pest management in the agro-industry. Particular emphasis is placed on stereo-selective synthesis of chiral pesticides and the application of their enantiopure formulations. The green aspects during chromatographic separation of enantiomers of chiral pesticides are also discussed. These include the use of green mobile and chiral stationary phases during chromatographic analysis of chiral pesticides.     

Total synthesis of citrafungin A

The antifungal natural product citrafungin A was synthesized using, as key steps, an asymmetric aldol reaction of a chiral oxazolidinone, diastereoselective alkylation of a chiral 1,3-dioxolan-2-one, semihydrogenation of an enyne, and selective methyl ester deprotection.     

A simple, short, and flexible synthesis of viridiofungin derivatives

Described herein is a simple, flexible, and efficient synthesis of the skeleton of the viridiofungins, a family of microbial secondary metabolites. The synthesis utilizes an asymmetric aldol reaction of a chiral oxazolidinone, a diastereoselective alkylation of a chiral 1,3-dioxolan-2-one, and a geometrically selective alkene cross-metathesis reaction as the key C-C bond-forming steps.     

The Resolution of Enantiomeric Ibuprofen on Chiral Stationary Phases Containing N-Arylcarbamoyl Derivatives of Phenylglycine

A series of fourteen anilide derivatives of ibuprofen were resolved on six chiral stationary phases (CSPs) derived from N-arylcarbamoyl derivatives of (S)-phenylglycine. Excellent chiral resolutions were achieved on these CSPs. The ionic-type CSPs showed better chiral recognition abilities than the corresponding covalent-type CSPs, and the CSP bearing two chiral centers has better performance than the CSPs bearing only one chiral center. The highest separation factor was achieved using the ionic-type CSP bearing two chiral centers for the resolution of the 3,5-dinitroanilide derivative of ibuprofen. This result is better than those reported in literature for the resolution of ibuprofen on the CSPs derived from amino acids, According to the chromatographic behaviors, the hydrogen bonding interaction, the π-π interactions provided by the phenyl groups in CSPs bearing one chiral center, and the phenylethylcarbamoyl moiety in CSPs bearing two chiral centers dominate the chiral recognition.     

Synthesis of new chiral pyrylium salts and their phosphinine and pyridine derivatives

Despite their versatility, chiral pyryliums are almost unknown in the literature. Reported here is the synthesis of several new chiral pyrylium salts and the corresponding pyridines and phosphinines. This work more than doubles the number of reported chiral pyryliums, and also represents the first racemizable/epimerizable pyryliums. The derived phosphinines and pyridines represent rare α-chiral ligands for transition metals.     

A Model Study of Intramolecular Asymmetric Radical Cyclizations of α-Ester and α-Amide Radicals

Starting from malonate, a practical route was developed for the synthesis of α-phenylthio acid 3. Several chiral compounds including (-)-menthol, (-)-8-pbenylmenthol and a camphor based oxazolidinone 8 reacted with 3 to give α-phenylthio esters or amide. These sulfides cyclized efficiently when reacted with tributyltin hydride. Among the chiral auxiliaries used, 8-phenylmenthyl group displayed moderate asymmetric induction (64% ee for cis-product and 40% ee for trans-product). Based on this results, a transition state model was proposed to explain the observed stereoselectivity. In this model, due to π,π-orbital overlap of the phenyl ring and the carbonyl, the si-face of the most stable conformer of the radical was shielded. This controlled the carbon-carbon bond formation to occur from the re-face.     

Asymmetric synthesis of α,β-substituted γ-amino acids via conjugate addition

The first conjugate addition reaction of organocuprates to N-enoyl oxazolidinone where a N-protected γ-nitrogen atom and an α-methyl group are present into α, β-unsaturated system is described. This reaction gave anti-products in moderate yields and high diastereomeric ratios. The anti-products have two contiguous stereogenic centers, one formed by the conjugate addition reaction and the other by a diastereoselective protonation reaction. The removal of chiral oxazolidinone moiety and N-deprotection of amino group furnished chiral α, β-disubstituted γ-amino acids.